The invention relates to a charge coupled device with a semiconductor body which is provided at a surface with a system of silicon electrodes to which voltages can be applied for controlling the storage and transport of electric charges in the semiconductor body, which electrodes are separated from the surface at least locally by a dielectric layer comprising a dual layer of silicon oxide and silicon nitride, the latter being locally provided with openings. The invention also relates to a method of manufacturing such a charge coupled device.
A charge coupled device and a method of the kind as described above are known inter alia from U.S. Pat. No. 4,077,112. A device is described therein where the electrodes are formed in two or more layers of polycrystalline silicon (referred to as poly hereinafter). In a first step, the (active portion of the) surface of the semiconductor body is covered with gate oxide on which subsequently a layer of silicon nitride is deposited. The first poly layer is provided thereon, and a number of electrodes are formed from the poly layer by known photolithographic means. Said electrodes are covered with a layer of silicon oxide by thermal oxidation, this layer forming an electrical insulation layer against any subsequent poly layer. During the oxidation step, the portion of the surface not covered with poly is masked against oxidation by the silicon nitride layer, whereby it is prevented that the gate oxide becomes thicker locally than is desired. After the oxidation, openings are formed in the silicon nitride layer through etching in a self-aligned manner, the poly electrodes formed with the oxide layer present thereon acting as an etching mask during this. As is described in the Patent, these openings in the nitride layer are necessary for efficiently passivating the surface in a later stage through annealing in hydrogen. This is because the density of silicon nitride is usually so great that, without openings in the silicon nitride layer, the hydrogen would be incapable of reaching the surface of the semiconductor body everywhere. A light oxidation step is subsequently carried out which renders the thickness of the uncovered gate oxide somewhat greater. Then a second conductive layer is deposited from which electrodes are formed next to the electrodes already present and electrically insulated therefrom by the oxide layer on the electrodes which were provided first.
As was noted above, the surface of the semiconductor body can be effectively passivated through heating in an environment containing hydrogen, during which hydrogen can spread through the openings in the nitride over the surface through diffusion. This step considerably reduces the concentration of surface states, and thus the leakage current. A reduction in the leakage current is of particular, though not exclusive importance in charge coupled imaging devices because local leakage currents may give rise to a greater spread in the dark current, and thus to an uneven picture on a display device.
The present invention has for its object inter alia to reduce the leakage current still further.
According to the invention, a charge coupled device of the kind described in the opening paragraph is for this purpose characterized in that the openings in the silicon nitride layer are bounded by edges which, seen transverse to the surface, lie at a distance from the edges of adjoining electrodes. The invention is based inter alia on the recognition that an oxide with a LOCOS-type structure is formed in the openings in the silicon nitride layer in the known device. The transition from this LOCOS oxide to the original, thinner gate oxide via a so-called bird""s beak, the location of which is defined by the edges of the openings in the silicon nitride layer, coincides with the edges of the poly electrodes which are present. The invention is furthermore based on the recognition that the presence of the poly electrodes above the bird""s beak may induce additional mechanical stresses in the subjacent active region, which stresses cause an increase in the leakage current or, in the case of an imaging device, the dark current. In a charge coupled device according to the invention, the edges of the poly electrodes do not coincide with the edges of the openings in the nitride layer, so that major leakage currents are avoided.
The invention offers particular advantages when the surface area of the charge coupled device is very large, because in such cases the openings in the silicon nitride layer are important for the lateral spread of the hydrogen over the entire device. A major embodiment according to the invention is characterized in that the device comprises a number of parallel charge transport channels which form a matrix in which charge packets can be stored in a bidimensional pattern and can be transported in parallel to read-out means.
The device may be, for example, a memory of the SPS type here where (digital) information is put in in series, is transported in parallel through the matrix, and is read out in series again.
An embodiment for which a low and uniform leakage current is particularly important, because non-uniformities in the leakage current will become visible again upon the display of the image, is characterized in that the device forms a charge coupled imaging device.
A preferred embodiment of a charge coupled imaging device according to the invention is characterized in that the system of electrodes forms openings, referred to as windows, which transmit electromagnetic radiation, within which the surface is not covered by material of the electrodes, and which are situated between mutually adjoining charge transport channels. Since the electromagnetic radiation can reach the photosensitive portion of the device through said windows without passing through the poly material of the In electrodes, the sensitivity of the imaging device will be high. A further embodiment is characterized in that the openings in the silicon nitride layer are formed at the areas of said windows, the edges of the openings lying at a distance from the edges of the windows. Preferably, the distance between the edges of the openings in the silicon nitride layer and the edges of the windows is chosen to be equal to or greater than 1.0 xcexcm so as to minimize the influence of the edges of the electrodes on the bird""s beak and thus to minimize the leakage current. Since there will always be a certain increase in the leakage current at the areas of the openings in the silicon nitride layer, it is advantageous to drain off this leakage current as much as possible through channel bounding zones. Accordingly, a further embodiment is characterized in that mutually adjoining charge transport channels in the semiconductor body are separated from one another substantially only by a channel bounding region, the openings in the silicon nitride layer being situated above the channel bounding regions.
A further embodiment of a charge coupled imaging device according to the invention, in which the openings in the silicon nitride layer can be provided independently of the positions of radiation-transmitting windows in poly layers, is characterized in that at least a number of the openings in the silicon nitride layer is provided between the surface of the semiconductor body and electrodes which extend over the openings. If the electrodes in this embodiment are provided with radiation-transmitting windows, the silicon nitride can also be removed at the areas of these windows through the use of a mask. A preferred embodiment, which has the advantage that no oxidation is necessary at the areas of these windows, is characterized in that the surface is covered by portions of the silicon nitride layer at the areas of the radiation-transmitting windows.
Various designs are possible when the electrodes are manufactured from two or more poly layers. Thus the openings may be formed in the silicon nitride layer before the deposition of the first poly layer or after the deposition of the first poly layer and before deposition of a second or third poly layer. An important embodiment of the invention is characterized in that the electrodes are formed from two or more layers of silicon, electrodes formed from a first silicon layer being separated from the surface by a first layer of silicon nitride provided with said openings, and electrodes formed from a second layer of silicon being insulated from the electrodes of the first layer of silicon by a second silicon nitride layer which is also provided with openings.
A method according to the invention for manufacturing a device as described above is characterized in that the openings in the silicon nitride layer are defined by means of an etching mask which was formed by way of photolithography and is provided with openings through which the silicon nitride layer is subjected to an etching treatment.